Corals Have A Symbiotic Relationship With – Zooxanthellae

Do you ever look at a thriving reef tank and wonder how those vibrant, stony structures manage to grow so fast? It feels like magic, but it is actually a masterpiece of biological engineering that every hobbyist should understand.

In this guide, I promise to show you exactly how corals have a symbiotic relationship with microscopic organisms to thrive. By the end of this article, you will have the practical knowledge to maintain this delicate balance in your own home aquarium.

We are going to preview the science behind coral nutrition, the role of lighting, and how you can prevent the dreaded “bleaching” event. Let’s dive into the fascinating world of zooxanthellae and your reef’s health!

The Heart of the Reef: How Corals Have a Symbiotic Relationship With Algae

At its core, the secret to a healthy coral is not just the water or the salt; it is the resident living inside the coral’s tissue. Most reef-building corals have a symbiotic relationship with a type of algae known as zooxanthellae.

These tiny, single-celled organisms live within the coral polyps themselves. It is a classic “win-win” scenario that has existed for millions of years in our oceans.

The coral provides the algae with a protected environment and the compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral remove waste products like carbon dioxide.

The Energy Exchange

Think of the zooxanthellae as tiny solar panels living inside your coral. They capture light energy and turn it into glucose, glycerol, and amino acids.

In many cases, these algae provide up to 90% of the coral’s nutritional requirements. This is why lighting is the most critical factor for most reef-keepers.

Without this constant flow of energy, the coral cannot build its calcium carbonate skeleton. This is the foundation of the entire reef structure in your tank.

Waste Management and Recycling

Corals are masters of recycling. The waste products of the coral polyp—mainly nitrogen and phosphorus—act as fertilizer for the algae.

This internal recycling loop allows corals to thrive in nutrient-poor tropical waters. In our aquariums, we try to mimic this by keeping “clean” but “fed” water.

It is a beautiful, self-sustaining loop when it is in balance. Corals have a symbiotic relationship with these algae that ensures neither party goes hungry in the vast ocean.

Lighting: Powering the Symbiotic Engine

Since we know that corals have a symbiotic relationship with photosynthetic algae, we must treat our aquarium lights as the primary “food source.”

If your light is too weak, the algae cannot produce enough sugar to feed the coral. If the light is too intense, it can actually damage the relationship, leading to stress.

Don’t worry—getting your lighting right is easier than it sounds if you follow a few basic rules about PAR and spectrum.

Understanding PAR (Photosynthetically Active Radiation)

PAR is a measure of how much “useful” light is reaching your corals. For most soft corals and LPS (Large Polyp Stony), a PAR of 75-150 is usually perfect.

SPS (Small Polyp Stony) corals, like Acropora, usually require much higher levels, often between 250 and 400 PAR. They have a higher demand for energy to build their dense skeletons.

Investing in a PAR meter or renting one from a local club is the best way to ensure your symbiotic partners are getting exactly what they need.

The Importance of the Blue Spectrum

Zooxanthellae are particularly efficient at absorbing light in the blue and violet spectrum (400-500nm). This is why reef tanks often look very blue to the human eye.

While “white” light helps us see the natural colors of the fish, the blue light is what truly drives the growth of the algae inside the coral.

Modern LED fixtures allow you to tune these channels. Always prioritize the blue spectrum for the health of the coral-algae bond.

Beyond Algae: Other Symbiotic Partners in the Tank

While the relationship with algae is the most famous, it isn’t the only one. Corals have a symbiotic relationship with a variety of other organisms that help them survive.

In a home aquarium, observing these interactions is one of the most rewarding parts of the hobby. It creates a truly “living” ecosystem rather than just a collection of animals.

Let’s look at a few common examples you might want to introduce to your own tank setup.

Acro Crabs and Trapezia Crabs

If you keep SPS corals, you might find tiny crabs living among the branches. These are often “commensal” or “mutualistic” partners.

The crabs protect the coral from predators like the Crown-of-Thorns starfish. In exchange, the coral provides a safe home and feeds the crab with its mucus.

I always get excited when I see a “hitchhiker” crab that is reef-safe. They act like little janitors, keeping the interior of the coral colony clean of debris.

Clownfish and Their Hosts

We can’t talk about symbiosis without mentioning the clownfish and the anemone (which is a close relative of coral). While not a coral itself, the dynamic is very similar.

The clownfish provides the anemone with nutrients (through waste) and protection from butterflyfish. The anemone provides the clownfish with a stinging shield against predators.

Some hobbyists even see clownfish “hosting” in large fleshy corals like Euphyllia (Torch or Hammer corals). It is a sight to behold, though it can sometimes stress the coral polyps.

Coral Bleaching: When the Relationship Breaks Down

One of the most heartbreaking sights for a reefer is coral bleaching. This happens when the corals have a symbiotic relationship with their algae that is suddenly severed.

When a coral is stressed—usually by heat, light shock, or poor water chemistry—the zooxanthellae actually become toxic to the coral.

To survive, the coral polyp expels the algae into the water column. This leaves the coral transparent, revealing the white calcium carbonate skeleton underneath.

Common Causes of Bleaching in Aquariums

• Temperature Spikes: Even a 2-3 degree rise above the norm for an extended period can trigger bleaching.
• Alkalinity Swings: Corals love stability. A sudden jump in KH can shock the system.
• Light Shock: Moving a coral from a shaded area to a high-PAR area too quickly can cause it to “panic” and eject its algae.

If you notice a coral turning pale, don’t panic! Check your parameters immediately. If you catch it early, the coral can often re-absorb algae and recover its color.

How to Help a Bleached Coral Recover

First, move the coral to a lower-light area of the tank. Since it has lost its “solar panels,” high light will only cause further oxidative stress.

Second, ensure your water parameters are rock-solid. Stability is the best medicine for a stressed animal.

Third, consider target feeding. Since the coral isn’t getting energy from the sun, it needs to catch food (like reef roids or baby brine shrimp) to stay alive.

Feeding Your Corals: Supplementing the Relationship

While the algae provide the “sugar,” corals still need “protein” to grow and repair tissue. They are, after all, animals with mouths and stinging cells (nematocysts).

In a closed aquarium system, the natural supply of plankton is often limited. This is where you, the aquarist, come in to lend a hand.

Regular feeding can significantly increase the growth rate and color of your corals. It makes the corals have a symbiotic relationship with you as their caretaker!

Broadcast Feeding vs. Spot Feeding

Broadcast feeding involves putting liquid or powdered food into the water flow. This is great for feeding the whole tank at once, especially for small-polyp corals.

Spot feeding involves using a pipette or “turkey baster” to gently blow food directly onto the coral polyps. This is much more efficient for LPS corals with large mouths.

I recommend spot feeding once or twice a week. It is a great way to observe your corals closely and check for any signs of pests or disease.

The Best Foods for Reef Tanks

There are many great options on the market today. Look for foods that contain a mix of phytoplankton and zooplankton.

Amino acid supplements are also highly effective. They don’t necessarily feed the coral “calories,” but they provide the building blocks the coral needs to build tissue and pigments.

Just be careful not to overfeed! Excess food can lead to high nitrates and phosphates, which can fuel nuisance algae growth that competes with your corals.

Water Chemistry: The Foundation of Symbiosis

To keep the bond between coral and algae strong, the water must be perfect. Corals are sensitive to changes in their environment because they are “osmoconformers.”

This means their internal chemistry is heavily influenced by the water around them. If the water is off, the relationship inside their cells will suffer.

Focusing on the “Big Three” parameters will give your corals the best chance at a long and healthy life.

Alkalinity, Calcium, and Magnesium

Alkalinity (dKH): This is arguably the most important parameter. It provides the “building blocks” for the coral’s skeleton. Aim for a stable number between 8 and 10 dKH.

Calcium: Corals use calcium to build their stony structures. Keep this between 400 and 450 ppm.

Magnesium: This is the “glue” that keeps calcium and alkalinity from precipitating out of the water. Aim for 1300 to 1400 ppm.

By keeping these three stable, you ensure that the coral has enough energy and material to grow alongside its symbiotic algae.

Frequently Asked Questions (FAQ)

Do all corals have a symbiotic relationship with algae?

No, not all of them! While most of the corals we keep in reef tanks are “zooxanthellate” (meaning they have algae), there are also “azooxanthellate” corals.

Corals like Sun Corals (Tubastrea) or Gorgonians often live in deep water or caves where there is no light. They must catch 100% of their food to survive.

Can a coral survive if it loses its algae?

Yes, but only for a short time. This is what we call “bleaching.” If the conditions that caused the stress are fixed quickly, the coral can recover.

However, if the coral stays bleached for too long, it will eventually starve to death because it lacks its primary energy source.

Why do my corals look more colorful under blue light?

This is due to fluorescence. Corals produce special proteins that absorb high-energy blue light and re-emit it as lower-energy colors like green, orange, or red.

These pigments often act as a “sunscreen” for the symbiotic algae, protecting them from being overwhelmed by too much light energy.

How can I tell if my coral is getting too much light?

If a coral is getting too much light, it may look very pale or even turn white on the top surfaces. This is a sign the algae are being over-worked.

Conversely, if a coral is turning brown, it might be producing more algae to try and capture what little light is available. It’s all about finding that “Goldilocks” zone.

Conclusion: Success Through Understanding

The fact that corals have a symbiotic relationship with microscopic algae is one of the most incredible wonders of the natural world. It allows them to build massive reefs that can be seen from space!

As an aquarist, your job is to be the guardian of this relationship. By providing stable water parameters, the right spectrum of light, and occasional supplemental feeding, you can watch these animals thrive.

Remember, a reef tank is a marathon, not a sprint. Take your time, observe your corals daily, and enjoy the incredible biological partnership happening right in your living room.

If you have questions about your specific setup or are seeing signs of bleaching, don’t hesitate to reach out to the community here at Aquifarm. We are all learning together!